The invention relates to a method of and an apparatus for the quick and continuous segregation of radioactive iodine isotopes present in different fluid samples and especially in those taken from the primary coolant of the water-water-type nuclear power plants base on pressurized water reactors, primarily for analytical purposes. The radioactive concentration of iodine isotopes--being of high importance from technological and/or operational safety aspects--can directly be measured by means of the method proposed by the present invention and using a simple measuring method applied to liquid samples treated in the equipment realizing said method.
The radioactive iodine isotopes in the primary water circuit of the water-water-type nuclear power plants are fission products of the fuel. Their quantity (radioactive concentration), further the relative amounts of the various iodine isotopes with respect to each other, reliably indicate inhermeticity of the cladding of fuel elements, and sensibly follow the service condition of the reactor in its steady-state and transient periods of operation. Consequently, the qualitative and quantitative analysis of iodine isotopes constitute an important item of power-plane radio-analytics. The analysis--considering the above tasks--should be performed within shortest time possible, in continuous service and with good selectivity and sensitivity. A few theoretical and practical methods for the analysis of this kind are known for the isotopes .sup.131 I, .sup.132 I, .sup.133 I, .sup.134 I and .sup.135 I occurring in water-water-type nuclear power plants.
Soviet authors, V. V. Aksionov et al. have proposed in 1982 a method and equipment offering a partial solution of the above task. (Radiatsionnaya bezopasnost'i zashchita AES, Vol. 7. Energoisdat, Moscow, 1982; referred to in the INIS IAEA--in the International Nuclear Information System of the International Atomic Energy Agency, Vienna--under the number 15:014268). Their method is suitable for continuous detection of the radioactive concentrations of the .sup.132 I and .sup.134 I isotopes. By means of a special device, the intensity of gamma photons are measured, applying in the primary-circuit piping section two NaI(T1) scintillation detectors operating in the given energy inverval in gated mode, making use of the emission of cascade gamma photons by isotopes .sup.132 I and .sup.134 I. Correspondingly, the detectors are connected in coincidence connection to a suitably selected electronic signal processing unit. The measuring point is arranged between the cation and anion exchange resin columns of the water-cleaning equipment having its circuit partially arranged in the primary circuit, in order to reduce radioactivity resulting from the cations. The interference effects of some isotopes of short half-period (such as .sup.16 N, .sup.17 N) is eliminated by imposing a delay on the investigated samples, inserting a 10-minute by-pass section. By means of this method, the activity of .sup.132 I and .sup.134 I isotopes can be selectively measured, and a few per cent of overall activity is claimed as sensitivity limit.
T. Bereznai et al. (Energia es Atomtechnika, Budapest, 30, 1977, p. 38.) published method of and an apparatus equipment for solving the assumed radio-analytical tasks occurring in the PAKS Nuclear Power Plant of Paks (Hungary). The aim of the method is to provide continuous analyzis of the nuclides present in the primary-circuit coolant of the ractor, the determination of iodine isotopes being only a part of this overall task. The equipment incorporates detector systems permitting continuous measurements at two points, one at the inlet of the by-pass section serving for the specific purpose of the investigation, and the other at the absorbent placed into the flow of gases separated from the sample and drained. In the course of processing the sample, the iodine isotopes are bound by a "iodine filter" which has not been specified.
Aksionov and this co-workers claim their method to be suitable only for analyzing the two iodine isotopes mentioned above, whereas in the course of measurements aimed at checking reactor operation and safety, quantitative analysis of as many of the entire set of iodine isotopes as possible, preferably all of them, would be required. A further deficiency of the method lies in the unsatisfactory degree of obtainable selectivity and sensitivity. From among the detectors performing continuous measurements, incorporated in the equipment realizing the method developed by Bereznai and his co-workers, the detector directly measuring gamma-radiation of unprocessed samples is only suitable for the determination of the so-called matrix activity of isotopes representing the overwhelming part of radioactive concentration in the sample, and also its sensitivity to detect iodine isotopes is very poor. Any continuous measurement of the iodine content of the iodine filter would only ppermit the plotting of cumulated (integrated) radio-activity, and also the sensitivity of continuous measurements would be effected by the presence of effluent passing across the iodine filter and still containing a considerable part of matrix form activity.
In the course of developing the present invention, the aim has been to determine, in shortest time possible and continuously, the activity of iodine isotopes or some quantity proportional to their activity, in steady-state and transient operating conditions (i.e. during shut-down or start-up periods) of the reactor, said iodine isotopes being of outstanding importance from among all radioactive isotopes present in the primary coolant of the pressurized water nuclear reactors.